learnRuby.rb

class MyLogger
  private_class_method :new
  @@logger = nil
  def MyLogger.create
    @@logger = new unless @@logger
    @@logger
  end
end

=begin
log1 = MyLogger.create
log2 = MyLogger.create

puts log1.object_id
puts log2.object_id
=end

class Accounts
  def initialize(checking, savings)
    @checking = checking
    @savings = savings
  end

  private
  def debit(account, amount)
    account.balance -= amount
  end
  def credit(account, amount)
    account.balance += amount
  end

  public
  def transfer_to_savings(amount)
    debit(@checking, amount)
    credit(@savings, amount)
  end
end

class Account
  attr_reader :cleared_balance
  protected :cleared_balance
  def greater_balance_than(other)
    return @cleared_balance > other.cleared_balance
  end
end

# a string can be frozen
a = "s"
a.freeze

# Singleton method
class SingletonTest
  def info
    puts 'This is a singleton method'
  end

  def meth1
    puts 'This is method1'
  end

  def meth2
    puts 'This is method2'
  end
end

o1 = SingletonTest.new
o2 = SingletonTest.new

class << o2
  def meth1
    puts "This is o2's method1"
  end

  def meth2
    puts "This is o2' method2"
  end
end

#o1.meth1
#o2.meth1

# class variable
class Person
  @@number = 0

  def Person.getNumber_1
    @@number
  end

  # in Ruby, all the classes are objects too. So they can be treaed like Class's instance
  def self.getNumber_2
    @@number
  end

  class << Person
    def getNumber_3
      @@number
    end
  end

  class << self
    def getNumber_4
      @@number
    end
  end
end

#puts Person.getNumber_1
#puts Person.getNumber_2
#puts Person.getNumber_3
#puts Person.getNumber_4

# redefine class's method
=begin
class RedefTest
  def meth
    puts 'This is meth'
  end
end

RedefTest.new.meth

class RedefTest
  def meth
    puts 'This is new meth'
  end
end

RedefTest.new.meth

class RedefTest
  undef_method :meth
end

RedefTest.new.meth
=end

# when declare a class variable in module, all classes which include this module can access this variable
module TestModule
  @@foo = 10
end
class Klass
  include TestModule
  p @@foo += 1
end
class Base
  include TestModule
  p @@foo += 1
end

# access a static variable from a class or module
class Meth
  PI = 3.14
end

def circle_area(arg)
  Math::PI * arg * arg
end

p circle_area 5

class Object
  STATIC = 123
end

p ::STATIC